ATMEGA640 datasheet(61/449 Pages) ATMEL

61 / 449 page

ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

Watchdog Reset

When the Watchdog times out, it will generate a short reset pulse of one CK cycle dura-

tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period

TOUT. See “Watchdog Timer” on page 53. for details on operation of the Watchdog

Timer.

Figure 29. Watchdog Reset During Operation

Internal Voltage

Reference

ATmega640/1280/1281/2560/2561 features an internal bandgap reference. This refer-

ence is used for Brown-out Detection, and it can be used as an input to the Analog

Comparator or the ADC.

Voltage Reference Enable

Signals and Start-up Time

The voltage reference has a start-up time that may influence the way it should be used.

The start-up time is given in Table 29. To save power, the reference is not always turned

on. The reference is on during the following situations:

When the BOD is enabled (by programming the BODLEVEL [2:0] Fuse).

When the bandgap reference is connected to the Analog Comparator (by setting

the ACBG bit in ACSR).

When the ADC is enabled.

Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the

user must always allow the reference to start up before the output from the Analog Com-

parator or ADC is used. To reduce power consumption in Power-down mode, the user

can avoid the three conditions above to ensure that the reference is turned off before

entering Power-down mode.

Note:

1. Values are guidelines only. Actual values are TBD.

Table 29. Internal Voltage Reference Characteristics

(1)

Symbol

Parameter

Condition

Min

Typ

Max

Units

VBG

Bandgap reference voltage

TBD

1.1

TBD

tBG

Bandgap reference start-up time

TBD

µs

IBG

Bandgap reference current

consumption

TBD

µA

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